As is well known, a microscope is an optical instrument used to view, examine and study very fine details of an object. The most critical component of the microscope is the microscope objective. The magnifying power and the resolution of the fine details of objects being examined are mainly dependent on the objective characteristics.
One of the common defects of the microscope objective is the lack of correction of field curvature. This arises from the fact that the majority of lens elements in a microscope objective have positive power, which in turn have inward curving fields. The inward curving field is to some extent offset by the use of negative lenses, which contribute outward curving field curvature and are required for correction of other aberrations, such as spherical aberration, astigmatism, distortion, coma, and chromatic aberration. However, since the overall power of the objective is positive, in normal achromat designs, the field of view retains an inward curvature. This inward curvature permits a clear image of an object under study only in the center of the field of view while objects at the periphery of the field are blurred. This problem was addressed to some extent by K. D. Sharma in Applied Optics, vol. 24 p.299 in an article entitled “Medium Power Microobjective: A New Design” which article is hereby incorporated by reference. The article discusses an arrangement of lenses where a negative lens helps to flatten the curvature of the field of view.
What is needed then is an optical arrangement of lenses that meets the requirements of magnification and numerical aperture (NA) which has a flat field and diffraction limited performance.